Solar control coatings having a layer stack of glass/Si3N4/NiCr/Si3N4 are known, where the metallic NiCr layer is the sole infrared (IR) reflecting layer in the coating. Unfortunately, while such layer stacks provide efficient solar control and are overall good coatings, they sometimes are lacking in terms of: (a) rate of manufacture; and (b) durability due to high compressive stress of the silicon nitride undercoat.
In particular, such solar control coatings which may be characterized by blue glass side reflective color require very thick silicon nitride undercoat layers in order to achieve the desired optical performance. The silicon nitride undercoat is typically from 600 to 900 Å thick. Unfortunately, the sputter-deposition rate of silicon nitride is very slow. As a result, production line speed for such coatings must typically be reduced significantly compared to other comparable coatings thereby leading to higher costs. Moreover, such a thick layer of silicon nitride (i.e., greater than about 600 Å) typically has very high compressive stress, which can lead to significant durability problems such as brush test failures.
In view of the above, it will be apparent to those skilled in the art that there exists a need for a solar control coating which can address and overcome one or both of the aforesaid disadvantages (i.e., high cost and durability).
An undercoat of titanium oxide (e.g., TiO2) (on the glass) and silicon nitride is known in the art. Unfortunately, a problem of adhesion exists when silicon nitride is placed over titanium oxide. This problem results in bad durability, especially after heat treatment when delamination tends to occur upon durability testing. Moreover, the indices “n” and “k” of titanium oxide tend to significantly change upon heat treatment, sometimes leading to drastic color changes due to heat treatment. Thus, it can be seen that an undercoat of titanium oxide/silicon nitride on glass is problematic and undesirable.
In certain example embodiments of this invention, one or more of the aforesaid problems can be addressed and/or overcome by replacing a thick silicon nitride undercoat with a dual layer undercoat. In certain example embodiments, the dual layer undercoat may include a layer of or including tin oxide (e.g., SnO2) on the glass surface and a layer comprising silicon nitride thereover. Tin oxide (any suitable stoichiometry) is advantageous in that it is relatively durable, and is a low stress material with excellent adhesion to glass. Moreover, the sputtering rate for tin oxide is much higher than that of silicon nitride. Thus, the aforesaid problems of high cost (due to slow deposition rate) and durability (due to high compressive stress) can be overcome through the use of tin oxide as a bottom portion of the overcoat.
Accordingly, the tin oxide portion of the undercoat allows the coating to be sputtered at a faster rate thereby reducing costs, and also allows part of the silicon nitride layer to be removed thereby reducing internal stress and improving durability. On the other hand, the silicon nitride portion of the undercoat is provided in order to prevent and/or reduce oxygen diffusion from the glass or tin oxide into the IR reflecting layer during heat treatment, thereby improving heat treatability.
In certain example embodiments of this invention, there is provided a coated article comprising: a glass substrate; a layer comprising tin oxide supported by the glass substrate and being located beneath any and all IR reflecting layer(s) of the coated article; a layer comprising silicon nitride provided on and contacting the layer comprising tin oxide; an infrared (IR) reflecting layer located over the layer comprising tin oxide and over the layer comprising silicon nitride; and a dielectric layer provided on the substrate over at least the IR reflecting layer.